KR20090026769A - Steel for forging, process for producing the same, and forged article - Google Patents

Abstract

A steel for forging which is for obtaining a forged article with excellent fatigue properties and in which inclusions have been reduced in size; and a useful process for producing the steel for forging. Also provided is a forged article (especially, solid type crankshaft) obtained from the steel for forging. It can have satisfactory fatigue properties and the inclusions therein have been reduced in size. The steel for forging is characterized in that the concentration of magnesium in solution in the steel is 0.04-5 ppm and that of aluminum in solution in the steel is 50-500 ppm.

Description

Forging steel, manufacturing method and forging thereof {STEEL FOR FORGING, PROCESS FOR PRODUCING THE SAME, AND FORGED ARTICLE}

The present invention relates to a forged steel obtained by using forging steel, a method for manufacturing the same, and a forging steel, and more particularly, to a forging steel obtained by miniaturizing inclusions present in the steel, and a method and a forging obtained by using the forging steel. will be. The forging manufactured by using the forging steel of the present invention is widely used in industrial fields, such as machinery, ships, electricity, etc., but is applied to the crankshaft used as a transmission member of the drive source for ships as a typical use example below The explanation proceeds mainly.

For example, a large crankshaft manufactured by using forging steel, which is a transmission member of a ship driving source, requires excellent fatigue characteristics that are less likely to cause fatigue failure even in a harsh use environment.

As a method of improving the fatigue characteristic of a crankshaft, Nonpatent literature 1 discloses using the technique in a machining surface, and improving the fatigue characteristic. Specifically, in Non-Patent Document 1, by employing the Roder Ruget (RR) method, the fatigue characteristics are remarkably improved over the crankshaft manufactured by the free forging method, or the cold roll processing is performed to improve the fatigue strength. Is disclosed.

In addition, Non-Patent Document 2 examines the improvement of the fatigue characteristics of low-alloy steel employed in a crankshaft for ships. Specifically, in Non-Patent Document 2, (1) inclusions tend to be a starting point of fatigue failure, and the tendency becomes remarkable as the strength of steel is increased, (2) fatigue strength decreases as the inclusion size increases, (3) Steel materials including elongated inclusions are disclosed in which anisotropy of fatigue strength tends to occur. And in order to improve the fatigue characteristic of a forging material, it is concluded that it is effective to make an inclusion shape spherical and to make a dimension small.

However, the above report does not disclose specific means for spherical shape of the inclusion shape and to reduce the size, and also does not disclose the kind or size of the inclusion to be controlled. Therefore, in order to implement | achieve the shape control of the inclusion effective for the improvement of a fatigue characteristic, it is thought that further examination is needed.

By the way, various methods have been proposed so far as the shape control method of inclusions in steel, and for example, Patent Document 1 discloses both sulfides and oxides as means for obtaining structural low alloy steels having excellent lamellar resistance and hydrogen-hydrogen organic cracking resistance. A method of reducing and also controlling the shape of the inclusions has been proposed. Specifically, in order to suppress the formation of Mn sulfide that inhibits lamellar resistance and hydrogen organic crack resistance, it is proposed to reduce the amount of S and the amount of O and to add Ca or Mg.

In addition, Patent Literature 2 discloses inclusions such as suppressing the production of M2S or Al ₂ O ₃ series inclusions that are continuous in hot rolling by Mg and Ca addition, changing the shape, and minimizing the shape. It is disclosed to perform shape control.

Patent Document 3 and Patent Document 4 disclose that increasing the surface fatigue strength and the tooth bending fatigue strength as a gear member by making the oxide inclusions ultrafine. Specifically, it has been proposed to produce MgO and MgO-Al ₂ O ₃ that are difficult to aggregate into oxide inclusions. Moreover, it turns out that when a part of MnS which is sulfide is set to (Mn * Mg) S, the elongation of an interference | inclusion is suppressed and the anisotropy of mechanical strength is reduced.

Patent Document 5 discloses that MnS, CaS, MgS, (Ca, Mn) S, (Ca, Mg, Mn) S are present as sulfides in order to obtain mechanical structural steel having excellent machinability. Patent Document 5 discloses that, in particular, when REM, Ca, and Mg are contained to control the form of sulfide, the anisotropy of mechanical properties is suppressed, and the machinability is higher than that of S-containing free cutting steel.

However, the shape control technique of these inclusions is not intended for forgings used in harsh environments such as transmission members of ship driving sources. Therefore, in order to raise the fatigue characteristic of a forging, it is calculated | required to establish and examine the original inclusion control method for the forging steel used for manufacture of a forging.

Non-Patent Document 1: "Improvement of Crankshaft Progress", Japanese Society of Marine Engineers, October 1974, Vol. 8, No. 10, p.54-59

[Non-Patent Document 2]: "Fatigue Strength Characteristics of High Strength Crankshaft Materials", Journal of the JIME, 2001, vol. 36, No. 6, p.385-390

Patent Document 1: Japanese Patent Application Publication No. 58-35255

Patent Document 2: Japanese Patent Application Publication No. 57-59295

Patent Document 3: Japanese Unexamined Patent Publication: Hei 7-188853

Patent Document 4: Japanese Unexamined Patent Publication: Hei 7-238342

Patent Document 5: Japanese Unexamined Patent Publication No. 2000-87179

The present invention has been made in view of the above circumstances, and an object thereof is to provide a forging steel having a finely divided inclusion for obtaining a forging having excellent fatigue properties and a useful method for producing such forging steel. An object of the present invention is to provide a forged product (particularly, an integrated crankshaft) in which inclusions capable of exhibiting good fatigue characteristics, which are obtained by using such forged steel, are further refined.

The forging steel of the present invention, which was able to achieve the above object, has a dissolved Mg concentration in the steel of 0.04 to 5 ppm (hereinafter, ppm represents "mass ppm") and a dissolved Al concentration in the steel of 50 to The point is that it is 500 ppm. In addition, "dissolved Al concentration" and "dissolved Mg concentration" mean the density | concentration of Al and Mg which exist as a solid solution element without forming a compound in steel, For example, secondary ion mass spectrometry (SIMS) Is the value measured by

In the forging steel of the present invention, the oxide-based inclusions included in (a) steel satisfy the following formulas (1) and (2), and (b) the cross-sectional area of the oxide-based inclusions included in steel. When A (μm ² ), the average value of √A is preferably 160 μm or less.

[Equation 1]

[Equation 2]

However, [MgO] and [Al ₂ O ₃ ] represent the content (mass%) of MgO and Al ₂ O _{3 in} the oxide inclusions, respectively.

Moreover, in order to ensure the outstanding strength and toughness generally as forging steels and forgings, it is preferable to satisfy the following component compositions.

"C: 0.2 to 1.0% (hereinafter,% represents" mass% "), Si: 0.05 to 0.6%, Mn: 0.2 to 1.5%, Ni: 4% or less (not including 0%), Cr: 0.5 to 4%, Mo: 0.1 to 1.5%, V: 0.005 to 0.3%, respectively, and the remainder is made of Fe and unavoidable impurities. ''

The present invention also includes a forging manufactured using the forging steel, and in particular, the integrated crankshaft manufactured using the forging steel of the present invention exhibits excellent fatigue characteristics.

On the other hand, the method of manufacturing the forging steel of the present invention means that the MgO concentration in the top slag in the molten steel treatment step is 5% or more, and the dissolved Al concentration in the steel is 50 to 500 ppm in manufacturing the forging steel. The point is to control Al concentration in molten steel so that it may become.

The present invention is configured as described above, and by adjusting the dissolved Mg concentration and the dissolved Al concentration in the steel, it is possible to control the form of the inclusions to be formed, and to provide a forging steel having finer inclusions. The forged product obtained by using such forged steel can be expected to have excellent fatigue characteristics, and is particularly useful as a large forged product such as a crankshaft used in ships.

1 is a graph showing the relationship between √A of the wavefront inclusions and the endurance limit ratio.

2 is a graph showing the relationship between the dissolved Mg concentration in the steel and the endurance limit ratio.

3 is a graph showing the effect of the concentration of (MgO + Al ₂ O ₃ ) and MgO in the oxide on the endurance limit ratio.

4 is a graph showing the effect of the dissolved Al concentration in the steel on the dissolved Mg concentration in relation to the MgO concentration in the slug.

5 is a graph showing the effect of dissolved Al concentration in steel and MgO concentration in top slag on the endurance limit ratio.

6 is a graph showing the relationship between the total Mg concentration in the steel and the endurance limit ratio.

The present inventors studied from various angles with the final goal of improving the fatigue characteristics of the forged product used in the harsh environment under the above circumstances. In particular, in large ingots (e.g., 20 tons or more) having a low solidification temperature, a target fatigue strength is hard to be obtained, and studies have been conducted from a different viewpoint.

As a result, the cause of the decrease in fatigue strength is a coarse inclusion having MgS as a main component, and the dissolved Mg concentration and dissolved Al concentration in steel are largely involved in the generation of this coarse inclusion. The present invention has been found to be able to suppress the formation of the coarse inclusions by controlling to within a predetermined range, thereby completing the present invention.

In addition, the present inventors examined the small ingot (20 kg) and the large ingot (2 tons), and it was found that the dissolved Mg concentration and the dissolved Al concentration in the steel were involved in the oxide composition regardless of the size of the ingot. In addition, by controlling these concentrations in an appropriate range, it has been found that formation of coarse inclusions such as Al ₂ O ₃ and CaO · Al ₂ O ₃ can be suppressed, and composition control can be performed with MgO-containing oxides which are difficult to aggregate. .

In order to control the dissolved Mg concentration in steel to an appropriate range, it turned out that what is necessary is just to adjust MgO concentration in top slag and Al concentration in molten steel suitably in a molten steel treatment process.

In the production method of the present invention, it is essential to properly adjust the MgO concentration in the top slag and the Al concentration in the molten steel in the molten steel treatment step, but the effect of the present invention is explained while explaining the basic procedure of the molten steel treatment step. Explain about.

In the molten steel treatment, first, a raw material is charged into an electric furnace and heated and melted, followed by decarburization and dephosphorization by oxygen blowing to blow oxygen from an oxygen lance. After the completion of the oxygen blow, the molten steel is moved to the ladle, and molten steel treatment is performed using a molten steel treatment apparatus such as LF (Ladle Furnace). At this time, prior to the molten steel treatment, slag raw materials such as CaO, MgO, Al ₂ O ₃ and the like are added to the molten steel surface at a predetermined mixing ratio and melted to form slag (top slag) on the molten steel surface.

In such molten steel treatment, while stirring molten steel by means, such as low odor gas stirring, temperature and a main component are adjusted, deoxidizer is added to molten steel, and deoxidation, desulfurization, etc. are performed. Moreover, if necessary, a vacuum degassing treatment is performed using a cover degassing apparatus, a tank degassing apparatus, and a circulating degassing apparatus (RH apparatus, etc.) to promote dehydrogenation and desulfurization from molten steel. The molten steel treatment process is completed at the stage where molten steel becomes a predetermined component, temperature, and cleanliness, and a steel ingot is cast by the coarsening method, such as a permanent column and a lower column.

The steel ingot obtained by the said ingot process is shape | molded in the shape of intermediate products, such as a round rod, by hot forging after that. After molding, the components, defects, cleanliness, etc. are subjected to an intermediate inspection, and then hot forged again to form a large product shape such as an integrated crankshaft or journal. Subsequently, after performing heat treatment according to the required product characteristics, it is finished by machining to become a final product.

The following process can be mentioned as a specific procedure for manufacturing an integrated crankshaft from the said steel ingot. That is, the solidified steel ingot is taken out from a mold and heated to 1150 degreeC or more. Thereafter, the steel sheet is processed into a round bar shape or a stepped shape by hot forging with a forging ratio of 3 or more. In this ingot forging, in order to compress an intrinsic defect, you may shorten to predetermined length after compressing in a ingot height direction. After hot forging, it is processed into the shape of an integrated crankshaft. In addition, in the forging of the integrated crankshaft, the slow parts may be molded one by one or a plurality of slow parts may be simultaneously molded by inserting the whole. After forming forging, machining for finishing is performed to obtain an integrated crankshaft having a predetermined dimension. Moreover, it is good also as an integral crankshaft by machining what was processed to step shape by hot forging.

In the method of the present invention, the manufacturing conditions of the above-mentioned large forged product, particularly in the molten steel treatment step, are controlled appropriately, and dissolved in steel by maintaining the top slag composition and the concentration of Al added as a deoxidizer in an appropriate range. It is to control the Mg concentration and the dissolved Al concentration. By controlling the dissolved Mg concentration and the dissolved Al concentration in an appropriate range, the oxides produced during the molten steel processing and casting are controlled to be easily dispersed, and the inclusions in the product after hot forging are finely made, resulting in a significant fatigue strength of the product. Will be improved. Hereinafter, each requirement prescribed | regulated by this invention is demonstrated.

In the method of the present invention, the MgO concentration in the top slag in the molten steel treatment step is 5% or more, and the Al concentration in the molten steel is adjusted so that the dissolved concentration in the steel is 50 to 500 ppm. By satisfying these requirements, the dissolved Mg concentration in the steel can be controlled to 0.04 to 5 ppm without directly adding Mg-containing alloys to molten steel (see FIGS. 4 and 5 to be described later). The top slag is usually composed of CaO-Al ₂ O ₃ -SiO ₂ -MgO-CaF ₂ , but the MgO concentration is a ratio with respect to all of them.

When Mg alloy is directly added to molten steel, a region having a high Mg concentration for a short time and locally may be formed in the molten steel ladle to produce coarse sulfides such as MgS. When this coarse sulfide aggregates with other inclusions and remains in a product, there exists a possibility that the fatigue characteristic of a forged product may remarkably fall.

When the dissolved Al concentration in the steel is less than 50 ppm, the dissolved oxygen amount increases, the number of oxides crystallized out during solidification increases, and the cleanliness deteriorates. When the dissolved concentration exceeds 500 ppm, the dissolved oxygen concentration decreases, and the dissolved Mg concentration in the steel increases to exceed 5 ppm (to be described later).

In order to control the dissolved Al concentration in the steel in the above-described range, the Al concentration in the molten steel is analyzed and the relationship between the Al concentration in the molten steel and the Al concentration in the steel is determined, and based on these, the final dissolved Al concentration in the steel is 50. Al or an Al alloy may be added in the molten steel so as to be in the range of from 500 ppm (to the Al concentration in the molten steel according to the concentration).

In the forging steel obtained by the above method, the dissolved Mg concentration in the steel is in the range of 0.04 to 5 ppm, and most of the deoxidation products present in the steel become MgO-containing oxides such as spinel, which significantly improves the fatigue strength of the steel. (See FIG. 2 and FIG. 4 to be described later). That is, when the dissolved Mg concentration in the forging steel is less than 0.04 ppm, the inclusion composition becomes Al ₂ O ₃ -rich, which causes agglomeration phenomenon. In addition, when the dissolved Mg concentration exceeds 5 ppm, MgS, MgO, and the like are generated in a large amount during solidification, resulting in coarse inclusions, thereby degrading cleanliness.

As described above, by setting the dissolved Mg concentration in the steel to an appropriate range, excellent fatigue characteristics of 0.42 or more are exhibited in the endurance limit ratio (fatigue strength (σ _W ) / tensile strength (σ _B )) described later. The preferable range of this dissolved Mg concentration is about 0.1 to 2 ppm, and by controlling in this range, more excellent fatigue characteristics (0.44 or more in the endurance limit ratio) are exhibited.

By the way, a wet analysis method is generally employed for the Al concentration and the Mg concentration in the steel, but the wet analysis cannot completely dissolve the oxides or sulfides, and accurately quantifies the concentration of Al and Mg dissolved as atoms in the steel. It is difficult. In addition, there is also a method of using soluble Al (sol. Al) as the concentration except Al in the oxide, but Al elution from CaO-Al ₂ O ₃ -based oxides cannot be ignored, so-called "sol.Al" is never accurately dissolved. It cannot be said that it is Al concentration. Therefore, in the present invention, SIMS is employed as an accurate measurement method of dissolved elements ("dissolved Al concentration" and "dissolved Mg concentration") from the viewpoint that the thermodynamic equilibrium of dissolved elements and oxides is very important for inclusion control.

In the forging steel of the present invention, it is preferable that the average composition of the oxide inclusions included in the steel satisfy the following formulas (1) and (2).

However, [MgO] and [Al ₂ O ₃ ] represent the content (mass%) of MgO and Al ₂ O _{3 in} the oxide inclusions, respectively.

By setting the oxide-based inclusion composition that satisfies the above formulas (1) and (2), the inclusions are made of MgO-containing oxides such as spinel or MgO. Since MgO-containing oxides have better wettability with molten steel than Al ₂ O ₃ , the occurrence of cohesive phenomenon of inclusions can be suppressed, and formation of coarse inclusions deteriorating fatigue characteristics of steel can be prevented.

In the forging steel of the present invention, when the cross-sectional area of the oxide inclusions contained in the steel is A (μm ² ), the average value of √A is preferably 160 μm or less (see FIG. 1 to be described later). When these requirements are satisfied, the inclusion size serving as the starting point for breaking the steel becomes small, and the fatigue strength and toughness in the final product (forged product) are improved. On the other hand, the presence of coarse sulfides and coarse oxides is undesirable for improvement of fatigue properties (see FIG. 2 to be described later).

The present invention is characterized in that the dissolved components in molten steel are adjusted in order to refine the inclusions in this way, and the basic composition of the forging steel is not particularly limited. However, in order to ensure the strength and toughness required as a crankshaft, etc., and the fatigue characteristic improvement aimed at the end of this invention, for example, it is preferable that steel materials satisfy | fill the following basic composition.

[C: 0.2% to 1.0%]

C is an element which contributes to strength improvement, and in order to ensure sufficient strength, it is preferable to contain C at least 0.2%, more preferably at least 0.3%, even more preferably at least 0.36%. However, if the amount of C is too large, the toughness is deteriorated, so it is 1.0% or less, more preferably 0.5% or less, and still more preferably 0.45% or less.

[Si: 0.05 to 0.6%]

Si acts as a strength improving element, and in order to ensure sufficient strength, it is preferable to contain Si at least 0.05%, more preferably at least 0.1%, even more preferably at least 0.2%. However, when there is too much Si amount, reverse V segregation becomes remarkable and a clean ingot becomes difficult to be obtained, It is good to suppress to 0.6% or less, More preferably, 0.4% or less.

[Mn: 0.2 to 1.5%]

Mn is also an element that increases the hardenability and contributes to the strength improvement, and in order to secure sufficient strength and hardenability, it is preferable to contain 0.2% or more, more preferably 0.4% or more, and still more preferably 0.9% or more. However, if the amount of Mn is too large, reverse V segregation is encouraged, so it is preferable to suppress the content to 1.5% or less, more preferably 1.2% or less, and still more preferably 1.1% or less.

[Ni: 4% or less (does not include 0%)]

Ni is an element useful as a toughness improving element, but if the amount of Ni is excessive, the cost increases, and therefore it is preferable to suppress it to 4% or less, preferably 2% or less.

[Cr: 0.5 to 4%]

Cr is an effective element which improves hardenability and improves toughness, and their effect is effectively exhibited by containing 0.5% or more, preferably 0.9% or more, and more preferably 1.5% or more. However, if the amount of Cr is excessively large, inverse V segregation is encouraged to make the production of high-clean steel difficult, and therefore it is preferable to suppress it to 4% or less, more preferably 2.5% or less.

[Mo: 0.1 to 1.5%]

Mo is an element which acts effectively for all the improvement of hardenability, strength, and toughness, and in order to exhibit these effects effectively, it is preferable to contain Mo 0.1% or more, More preferably, it is 0.15% or more, More preferably, it is 0.20% or more. Do. However, since Mo is easy to generate micro segregation (normal segregation) because the equilibrium distribution coefficient is small, it is preferable to suppress it to 1.5% or less.

[V: 0.005 to 0.3%]

V has an effect of strengthening precipitation and microstructure, and is a useful element for high strength. In order to exhibit such an effect effectively, it is recommended to contain V 0.005% or more. However, even if it contains excessively, since the said effect is saturated and it is economically wasteful, it is good to suppress it to 0.3% or less, More preferably, it is 0.15% or less.

Preferred basic components of the forging steel used in the present invention are as described above. Although the remainder is substantially Fe, the forging steel is allowed to contain a small amount of unavoidable impurities, and of course, it is possible to use forging steel which actively contains other elements within a range that does not adversely affect the operation of the present invention. It is also possible. Examples of other elements that can be positively added include B having a hardenability improving effect, Ti having a deoxidation effect, W, Nb, Ta, Cu, Ce, La, Zr, Te and the like which are solid solution strengthening elements or precipitation strengthening elements. Can be. Although they can add individually or in combination of 2 or more types, it is preferable to suppress to about 0.1% or less in total amount.

Moreover, although this invention also includes the forging obtained by using the said forging steel, the manufacturing method is not specifically limited. For example, a step of melting steel of a predetermined component composition in an electric furnace or the like → a step of removing impurity elements such as S or a gas component such as O by vacuum refining or the like → a step of ingoting → forging a material after heating the ingot The step of heating → forging into the shape of the product after the intermediate inspection → homogenizing by heat treatment, hardening by hardening, and then finishing machining may be performed in sequence.

In particular, when manufacturing a crankshaft as a forging product, when manufacturing as an integrated crankshaft, since the shaft surface layer side can be occupied in the part with high cleanliness, what is excellent in strength and fatigue characteristics is preferable. In this case, although the manufacturing method of an integrated crankshaft is not specifically limited, A preferable thing is R.R. And T.R. It is manufactured by the same method as the forging method (method of forging the shaft center of the ingot to become the center of the crankshaft axis and integrally forging the part that is likely to cause deterioration of characteristics due to segregation to the center of all the shafts of the crankshaft). .

As another forging method, it may be produced by a free forging method (a method of forging a block in which the crank arm and the crank pin are integrated and finishing the crankshaft by gas cutting and machining).

In addition, since the forging steel of the present invention exhibits excellent fatigue characteristics by minimizing inclusions, forging molding of high strength products such as slow, pressure vessels, hollow materials, etc. It can also be utilized effectively as a material for doing so.

Hereinafter, although an Example is given and this invention is demonstrated further more concretely, this invention is not restrict | limited by the following example of course, It is also possible to change suitably and to implement in the range suited for the purpose of the following, They are all included in the technical scope of the present invention.

20 to 100 tons of scrub material were melted in an electric furnace and cast out on the ladle. After that, crude agents such as CaO, Al ₂ O ₃ and MgO were added to the molten steel surface to form top slag having a predetermined composition. Then, the molten steel process was performed using the ladle heating type refinery apparatus provided with the low odor stirring apparatus. In this molten steel treatment step, Al was added to deoxidize the molten steel, and dehydrogenation was performed by vacuum treatment by a cover degassing apparatus. During the molten steel treatment, the molten steel was appropriately sampled to measure the Al concentration in the molten steel, and Al was further added as necessary so that the Al concentration was in a predetermined range.

At this time, the correspondence between the dissolved Al concentration in the steel analyzed by the secondary ion mass spectrometry (SIMS) by the preliminary experiment and the Al concentration in the molten steel analyzed by the luminescence analyzer in advance is known in advance, and the dissolved Al concentration in the steel is determined. The concentration in the molten steel was controlled to fall within the range.

After the molten steel treatment was completed, a sample of the top slag was taken, and the steel ingots (20 to 50 tons) were cast by the ingot casting method. After the solidification of the ingot was completed, the ingot was taken out from the mold, heated to 1150 ° C or higher, and hot forged to produce a round rod-shaped forged product of various sizes. At this time, hot forging was performed on the 20 ton ingot and finished in a round rod shape having a diameter of 400 to 500 mm, and hot forging was performed on the 50 ton ingot and finished in a round rod shape having a diameter of 500 to 600 mm.

The chemical composition of each forging was investigated by chemical analysis, and the MgO concentration was examined by ICP emission spectroscopy from the sample of the top slag. These results are shown in Table 1 together with the Al concentration in the molten steel.

The dissolved Mg concentration and dissolved Al concentration in the steel ingot were measured, and the inclusion composition analysis, the fatigue test, and the inclusion size measurement in the forged product were performed by the following method. At this time, the total Mg concentration (Total Mg concentration) in steel was also investigated by ICP-mass spectrometry (ICP-MS method).

[Measurement of dissolved Mg concentration and dissolved Al concentration in steel]

The sample taken from the ingot was polished, loaded into a secondary ion mass spectrometer (manufactured by "ims5f" CAMECA), and the secondary ion images of Mg and Al were observed in the region of 500 x 500 (µm ² ) for each sample. Then, three places where Mg and Al were not concentrated in the area were selected and analyzed in the depth direction. The primary ion source at this time is O ²⁺ . And when the density | concentration distribution of the depth direction is constant, the value was made into the dissolved concentration. In the case of inclusion in the course of the depth direction analysis, the concentration distribution is greatly changed. However, the analysis was carried out to the depth at which the inclusion does not exist and the concentration was constant. In addition, about the quantification method of density | concentration, the pure iron which ion-implanted ²⁴ Mg (150 keV, 1 * 10 <^14> atoms / cm <2>) and ²⁷ Al (200 keV, 1 * 10 <^14> atoms / cm <2>) as a standard sample was measured, and obtained Atomic concentrations were measured using relative sensitivity coefficients (RSF).

Inclusion Composition Analysis

In the round bar after forging, the sample was cut out from the center part of the ingot bottom equivalent position, and the component composition analysis of the inclusion by EPMA was performed. At this time, 50 or more inclusions were randomly selected for each sample, and the composition was analyzed and the average value thereof was obtained.

[Fatigue test and inclusion size measurement]

In the round bar after forging, the smooth test piece of diameter: 10 mm x length: 30 mm was extract | collected radially from the center part of the ingot bottom equivalent position, and the fatigue test was implemented on condition of the following. In addition, a tensile test was performed at room temperature using a test piece taken from the same position as the fatigue test piece. The endurance limit ratio (fatigue strength (σ _W ) / tensile strength (σ _B )) was measured as an index of the fatigue limit.

Test method: rotation bending fatigue test (stress ratio = -1, number of revolutions: 3600 rpm)

Fatigue Strength Evaluation Method: Method

Dependent stress: 20 MPa

Initial stress: 300 MPa

Number of test pieces: 5 each

Fatigue Strength for Each Test Specimen = (break stress)-(equal stress)

In addition, after a fatigue test, the fatigue wave surface was observed with the scanning microscope (SEM), the inclusion size which exists in the starting point of a fatigue fracture surface was measured, and 1/2 square of the cross-sectional area of an inclusion was calculated as (D). At this time, the presence or absence of the coarse inclusion containing MgS was also investigated.

Although these results are collectively shown in Table 2 below, in the case of satisfying the requirements specified in the present invention (steel Nos. 1 to 11), the inclusions are fined and the excellent endurance limit ratio (0.4 _W at σ _W / σ _B) is obtained. It turns out that the above) is achieved. On the other hand, from which either one is devoid of the requirements specified in the present invention (Steel No.12 to 17), without refinement of inclusions is not achieved, the endurance limit only the non-low value (less than 0.40 with σ _W / σ _B) It turns out that it is not obtained.

Based on the above results, the relationship between √A of the inclusions at the wavefront and the endurance limit ratio is shown in FIG. 1, but in order to improve the endurance limit ratio, it is effective that the size of inclusions (160 μm or less at √A) is effective. Able to know.

Although the relationship between the dissolved Mg concentration in the steel and the endurance limit ratio is shown in FIG. 2, it is found that when the dissolved Mg concentration in the steel exceeds a predetermined value (5 mass ppm), coarse sulfide (MgS) is formed and the endurance limit ratio is lowered. Can be. In addition, when the dissolved Mg concentration in steel is less than a fixed value (0.04 mass ppm), it is understood that an oxide (which is easy to aggregate) is formed (see Table 2 for composition thereof) and the durability limit ratio is lowered.

Figure 3 shows the effect of (MgO + Al ₂ O ₃ ) concentration and MgO concentration in the endurance limit ratio in the oxide. 3 is not plotted against Test Nos. 16 and 17 in which coarse sulfides were produced. As is apparent from this result, it can be seen that a high endurance limit ratio can be obtained by setting the (MgO + Al ₂ O ₃ ) concentration and the MgO concentration in the oxide to a predetermined value or more, respectively. Incidentally, in the case where a high endurance limit ratio is obtained, inclusions are naturally fine (see FIG. 1 above).

4 is a graph showing the effect of the dissolved Al concentration in the steel on the dissolved Mg concentration in relation to the MgO concentration in the slag. As is apparent from this result, when the MgO concentration in slag is less than 5%, it turns out that the target dissolved Mg concentration is not obtained. In addition, when MgO concentration in slag is 5% or more, it turns out that the target dissolved Mg concentration is obtained by controlling so that dissolved Al concentration in steel may be 50-500 mass ppm.

5 shows the effect of dissolved Al concentration in steel and MgO concentration in top slag on the endurance limit ratio. As is apparent from this result, it can be seen that a high endurance limit ratio can be obtained by controlling the MgO concentration in the top slag to 5 mass% or more and controlling the dissolved Al concentration in the steel in the range of 50 to 500 mass ppm.

Figure 6 shows the relationship between the total Mg concentration in the steel and the endurance limit ratio. As is apparent from this result, it can be seen that the correlation between the total Mg concentration and the endurance limit ratio is low, and that the dissolved Mg concentration in the steel is very effective to improve the endurance limit ratio (Fig. 2).

Claims (7)

A forging steel, wherein the dissolved Mg concentration in the steel is 0.04 to 5 ppm (hereinafter, ppm represents "mass ppm") and the dissolved Al concentration in the steel is 50 to 500 ppm. The forging steel according to claim 1, wherein the oxide inclusions contained in the steel satisfy the following formulas (1) and (2). [Equation 1]

[Equation 2]

However, [MgO] and [Al ₂ O ₃ ] represent the content (mass%) of MgO and Al ₂ O _{3 in} the oxide inclusions, respectively. The forging steel according to claim 1 or 2, wherein the average value of √A is 160 µm or less when the cross-sectional area of the oxide inclusions included in the steel is A (µm ² ). C: 0.2 to 1.0% (hereinafter% represents "mass%"), Si: 0.05 to 0.6%, Mn: 0.2 to 1.5%, Ni: 4 in any one of Claims 1-3. A forging steel comprising% or less (not containing 0%), Cr: 0.5 to 4%, Mo: 0.1 to 1.5%, and V: 0.005 to 0.3%, respectively, wherein the remainder is made of Fe and unavoidable impurities. A forged product manufactured by using the forging steel according to any one of claims 1 to 4. The forging according to claim 5, which is an integral crankshaft. In manufacturing the forging steel, the MgO concentration in the top slag in the molten steel treatment step is 5% or more, and the Al concentration in the molten steel is controlled so that the dissolved Al concentration in the steel is 50 to 500 ppm. Method of manufacturing forging steels.

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